Methods for selective functionalization and separation of carbon nanotubes

ABSTRACT

The present invention is directed toward methods of selectively functionalizing carbon nanotubes of a specific type or range of types, based on their electronic properties, using diazonium chemistry. The present invention is also directed toward methods of separating carbon nanotubes into populations of specific types or range(s) of types via selective functionalization and electrophoresis, and also to the novel compositions generated by such separations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/566,073, filed Jan. 26, 2006, now U.S. Pat. No. 7,572,426which is a 35 U.S.C. §371 National Stage entry of PCT Application serialno. PCT/US2004/024507, filed Jul. 29, 2004, which claims priority toU.S. Provisional Patent Application Ser. No. 60/490,755, filed Jul. 29,2003. These priority applications are incorporated by reference hereinin their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The present invention was made in with support from the Robert A. WelchFoundation, Grant No. C-0689; the National Aeronautics and SpaceAdministration, Grant Nos. NASA-JSC-NCC-9-77 and NASA TiiMS NCC-01-0203;the National Science Foundation, Grant Nos. DMR-0073046 and EEC-0118007;and the Air Force Office of Scientific Research, Grant No.F49620-01-1-0364.

FIELD OF THE INVENTION

The present invention relates generally to carbon nanotubes. Morespecifically, the invention relates to methods of selectivelyfunctionalizing carbon nanotubes by type, separating carbon nanotubes bytype, and populations of functionalized carbon nanotubes separated bytype to yield novel compositions.

BACKGROUND

Carbon nanotubes (CNTs), comprising multiple concentric shells andtermed multi-wall carbon nanotubes (MWNTs), were discovered by Iijima in1991 [Iijima, Nature 1991, 354, 56]. Subsequent to this discovery,single-wall carbon nanotubes (SWNTs), comprising a single graphenerolled up on itself, were synthesized in an arc-discharge process usingcarbon electrodes doped with transition metals [Iijima, S.; Ichihashi,T. Nature 1993, 363, 603; and Bethune et al. Nature 1993, 363, 605].These carbon nanotubes (especially SWNTs) posses unique mechanical,electrical, thermal and optical properties, and such properties makethem attractive for a wide variety of applications. See Baughman et al.,Science, 2002, 297, 787-792.

The diameter and chirality of CNTs are described by integers “n” and“m,” where (m,m) is a vector along a graphene sheet which isconceptually rolled up to form a tube. When |n−m|=3q, where q is aninteger, the CNT is a semi-metal (bandgaps on the order of milli eV).When n−m=0, the CNT is a true metal and referred to as an “armchair”nanotube. All other combinations of n−m are semiconducting CNTs withbandgaps in the range of 0.5 to 1.5 eV. See O'Connell et al., Science,2002, 297, 593. CNT “type,” as used herein, refers to such electronictypes described by the (m,m) vector (i.e., metallic, semi-metallic, andsemiconducting).

The main hurdle to the widespread application of CNTs, and SWNTs inparticular, is their manipulation according to electronic structure[Avouris, Acc. Chem. Res. 2002, 35, 1026-1034]. All known preparativemethods lead to polydisperse materials of semiconducting, semimetallic,and metallic electronic types. See M. S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press,San Diego, 1996; Bronikowski et al., Journal of Vacuum Science &Technology 2001, 19, 1800-1805; R. Saito, G. Dresselhaus, M. S.Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial CollegePress, London, 1998. Recent advances in the solution phase dispersion[Strano et al., J. Nanosci. and Nanotech., 2003, 3, 81; O'Connell etal., Science, 2002, 297, 593-596] along with spectroscopicidentification using bandgap fluorescence [Bachilo et al., Science,2002, 298, 2361] and Raman spectroscopy [Strano, Nanoletters 2003, 3,1091] have greatly improved the ability to monitor electrically distinctnanotubes as suspended mixtures and have led to definitive assignmentsof the optical features of semiconducting [Bachilo et al., Science,2002, 298, 2361], as well as metallic and semi-metallic species [Strano,Nanoletters, 2003, 3, 1091].

Techniques of chemically functionalizing CNTs have greatly facilitatedthe ability to manipulate these materials, particularly for SWNTs whichtend to assemble into rope-like aggregates [Thess et al., Science, 1996,273, 483-487]. Such chemical functionalization of CNTs is generallydivided into two types: tube end functionalization [Chen et al.,Science, 1998, 282, 95-98], and sidewall functionalization [PCTpublication WO 02/060812 by Tour et al.].

In view of the above, it would be particularly advantageous to have amethod that is capable of selectively functionalizing CNTs, and SWNTs inparticular, based on their electronic structure and/or properties.

SUMMARY

The present invention is directed toward a method of selectivelyfunctionalizing carbon nanotubes of a specific type or range of types,based on their electronic properties. The present invention is alsodirected toward methods of separating carbon nanotubes into populationsof specific electronic types or range(s) of types via a combination ofselective chemical functionalization and electrophoresis, and the novelcompositions generated by such separations. Optionally, these isolatedcompositions can be thermally defunctionalized to yield populations ofunfunctionalized, pristine carbon nanotubes of a specific electronictype or range of types.

The present invention provides the first selective reaction pathways ofcarbon nanotubes where covalent chemical functionalization is controlledby differences in the nanotube electronic structure. Such chemicalpathways provide for the manipulation of nanotubes of distinctelectronic types by selective functionalization of metallic nanotubes.Controlling nanotube chemistry in this way allows for the separation ofsemiconducting from metallic and semi-metallic nanotubes with highselectivity and scalability: a long sought goal of the carbon nanotubecommunity.

Generally, methods of the present invention that provide for selectivelyfunctionalized carbon nanotubes, and particularly single-wall carbonnanotubes, involve reaction of solvent-suspended carbon nanotubes withone or more diazonium species. By exploiting the differential reactivityof such diazonium species toward metallic and semi-metallic carbonnanotubes, addition of a substoichiometric amount of diazonium speciesto a mixture of carbon nanotubes of varying type results in only themetallic and semi-metallic carbon nanotubes being functionalized. Suchdiazonium species permit the metallic and semi-metallic carbon nanotubesto be functionalized with a variety of chemical moieties.

In general, methods for selectively functionalizing carbon nanotubescomprise the steps: a) selecting a quantity of carbon nanotube material;b) suspending the carbon nanotube material in a solvent; and c) adding achemical reactant that is able to selectively functionalize the carbonnanotube material based on the electronic properties of the nanotubes.Generally, the chemical reactant is added in a substoichiometric amount,and the reactant is typically a diazonium species.

In general, methods for separating carbon nanotubes on the basis oftheir electronic bandgap comprise the steps: a) functionalizing carbonnanotubes to yield a mixture of selectively-functionalizedsurfactant-suspended carbon nanotubes bearing phenol moieties, wherein aportion of the carbon nanotubes within the mixture have beenselectively-functionalized and another portion within the mixtureremains unfunctionalized; b) deprotonating the OH groups (on the phenolgroups) present in the mixture of selectively-functionalizedsurfactant-suspended carbon nanotubes by increasing pH; and c)electrophoretically separating the functionalized carbon nanotubes fromthe unfunctionalized carbon nanotubes.

The foregoing has outlined rather broadly the features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a reaction scheme, wherein (A) diazonium reagents extractelectrons, thereby evolving N₂ gas and leaving a stable C—C covalentaryl bond to the nanotube surface; (B) the extent of electron transferis dependent on the density of states in that electron density nearE_(F) leads to higher initial activity for metallic and semimetallicnanotubes; (C) the arene-functionalized nanotube may now exist as thedelocalized radical cation, which could further receive electrons fromneighboring nanotubes or react with fluoride or diazonium salts; and (D)a relative lack of electron density near E_(F) occurs in semiconductingcarbon nanotubes (|n−m|≠3q);

FIG. 2 depicts (A) UV-vis-nIR spectra of sodium dodecylsulfate-suspended carbon nanotubes after the addition of various amountsof 4-chlorobenzenediazonium tetrafluoroborate (in mol/1000 mol carbon),and wherein (B) is an expanded view of the metallic region, wherein thepeaks (a-f) are seen to decrease with increasing side groupconcentration;

FIG. 3 depicts (A) Raman spectrum at 532-nm excitation, showing thegrowth of the “disorder” mode with increasing functionalization from 0(i) to 5.6 (ii) to 22.4 (iii) groups attached per 1000 carbon atoms;wherein (B) the intensity of the tangential mode (TM)×0.1 decreases asresonance enhancement of the scattering event is lost with increasingreaction; and wherein the disorder mode, D, increases sharply thendecays because of the same loss of enhancement;

FIG. 4 depicts (A) low wavenumber Raman spectra at 532-nm excitation ofthe starting solution, wherein four metallic/semi-metallic nanotubes[(13,1), (9,6), (10,4), and (9,3)] are probed at this wavelength and onesemiconductor (9,2) via a radial mode sensitive to nanotube diameter,wherein (B) after 5.6 groups attached per 1000 carbons,functionalization disrupts this mode, as seen by the decay particularlyof the small-diameter metals, and providing initial evidence ofselective reactivity among metals provides a handle for separation ofthese species, and wherein (C) after a ratio of 22.4, all metallic modeshave decayed, leaving only the single semiconductor, in agreement withFIG. 2B;

FIG. 5 depicts Raman spectra at 633 nm probing both metals andsemiconducting nanotubes before reaction (solid line) and after recoveryand thermal pyrolysis (dotted line), wherein the reversibility of thechemistry implies that intrinsic electronic and optical properties ofthe pristine nanotubes can be recovered;

FIG. 6 depicts (A) the selective functionalization of metallic carbonnanotubes with phenol groups (added as a diazonium species) and theirdeprotonation at elevated pH; (B) an electrophoresis trace showingdifferential migration of unfunctionalized and phenol-functionalizedcarbon nanotubes; and (C) a comparison of the electrophoretic mobilitybetween the unfunctionalized and phenol-functionalized carbon nanotubes,made by scaling the applied electric field.

DETAILED DESCRIPTION

The present invention is directed toward methods by which carbonnanotubes can be chemically functionalized, in a selective manner,according to their precise electronic structure. The present inventionis also directed toward methods of separating carbon nanotubes intopopulations of specific electronic types or range(s) of types via acombination of selective functionalization and electrophoresis, and alsoto the novel compositions generated by such separations. Optionally,these isolated compositions can be thermally defunctionalized to yieldpopulations of unfunctionalized, pristine carbon nanotubes ofhomogeneous type.

The problem of separating carbon nanotubes based upon their electronicproperties has been around since their initial synthesis in 1991. Theproblem stems from the fact that all current methods of producing CNTsyield inhomogeneous product of varying diameters and chiralities—andhaving various electronic structures. While there have been recentreports of separating SWNTs based on their electronic properties, therehas been no successful demonstration of using electronic chemicalselectivity to accomplish this feat. In fact, electronic selectivityhas, up to now, not been demonstrated.

While not intending to be bound by theory, it is believed that theselective functionalization processes of the present invention involvesan exploitation of charge transfer stability at the nanotube sidewall todirect the selective reaction of certain electronic structures overothers. Such methods form a basis for manipulating and separating carbonnanotubes by their electronic structure via chemical means which, insome embodiments of the present invention, yields populations of carbonnanotubes having specific diameters, chiralities, and electronicproperties. In some or other embodiments, populations of carbonnanotubes having specifically-tailored ranges of diameters, chiralities,and electronic properties are produced.

Carbon nanotubes (CNTs), according to the present invention, include,but are not limited to, single-wall carbon nanotubes (SWNTs), multi-wallcarbon nanotubes (MWNTs), double-wall carbon nanotubes, buckytubes,fullerene tubes, tubular fullerenes, graphite fibrils, and combinationsthereof. Such carbon nanotubes can be of a variety and range of lengths,diameters, number of tube walls, chiralities (helicities), etc., and canbe made by any known technique including, but not limited to, arcdischarge [Ebbesen, Annu. Rev. Mater. Sci. 1994, 24, 235-264], laseroven [Thess et al., Science 1996, 273, 483-487], flame synthesis [VanderWal et al., Chem. Phys. Lett. 2001, 349, 178-184], chemical vapordeposition [U.S. Pat. No. 5,374,415], wherein a supported [Hafner etal., Chem. Phys. Lett. 1998, 296, 195-202] or an unsupported [Cheng etal., Chem. Phys. Lett. 1998, 289, 602-610; Nikolaev et al., Chem. Phys.Lett. 1999, 313, 91-97] metal catalyst may also be used, andcombinations thereof. Depending on the embodiment, the CNTs can besubjected to one or more processing steps. In some embodiments, the CNTshave been purified. Exemplary purification techniques include, but arenot limited to, those by Chiang et al. [Chiang et al., J. Phys. Chem. B2001, 105, 1157-1161; Chiang et al., J. Phys. Chem. B 2001, 105,8297-8301]. In some embodiments, the CNTs have been cut by a cuttingprocess. See Liu et al., Science 1998, 280, 1253-1256; Gu et al., NanoLett. 2002, 2(9), 1009-1013; Haddon et al., Materials Research SocietyBulletin, 2004, 29, 252-259. The terms “carbon nanotube” and “nanotube”will be used interchangeably herein.

While not intending to be bound by theory, the diversity in electronicstructure of CNTs arises from the unique quantinization of theelectronic wavevector of the 1-D system through the conceptual rollingof a graphene plane into a cylinder forming the nanotube [M. S.Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes andCarbon nanotubes, Academic Press, San Diego, 1996; R. Saito, G.Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon Nanotubes,Imperial College Press, London, 1998]. The vector in units of hexagonalelements connecting two points on this plane defines the nanotubechirality in terms of two integers: n and m. When |n−m|=3q or zero,where q is an integer, the nanotube is metallic or semi-metallic, whilethe remaining species are semi-conducting with a geometry-dependentbandgap [Reich et al., Physical Review B, 2000, 62, 4273-4276]. Althoughlargely unrealized in previous studies, subtle differences in thegeometric structure of carbon nanotubes lead to dramatic changes in therates of solution phase reactivity of these species. Applicants havefound that water-soluble diazonium salts [Bravo-Diaz et al., Langmuir,1998, 14, 5098], which have been shown to react with carbon nanotubes[Bahr et al., J. Mat. Chem., 2002, 12, 1952-1958; Dyke et al., J. Am.Chem. Soc., 2003, 125, 1156; Bahr et al., J. Am. Chem. Soc., 2001, 123,6536-6542], and nanotubes that are surfactant-wrapped [Dyke et al., NanoLett., 2003, 3, 1215-1218] can extract electrons from nanotubes in theformation of a covalent aryl bond (FIG. 1A) [Dyke et al., SyntheticLett., 2004, 155-160] and thereby demonstrate superb chemoselectivereactions with metallic tubes over the semiconducting tubes. Referringto FIG. 1, (A) diazonium reagents extract electrons, thereby evolving N₂gas and leaving a stable C—C covalent aryl bond to the nanotube surface;(B) the extent of electron transfer is dependent on the density ofstates in that electron density near E_(F) leads to higher initialactivity for metallic and semimetallic nanotubes; and (C) thearene-functionalized nanotube may now exist as the delocalized radicalcation, which could further receive electrons from neighboring nanotubesor react with fluoride or diazonium salts. See Dyke et al., SyntheticLett., 2004, 155-160; Strano et al., Science, 2003, 301, 1519.

The above-described bonding forms with extremely high affinity forelectrons with energies, ΔE_(r), near the Fermi level, E_(f), of thenanotube (FIG. 1B). Again, while not intending to be bound by theory, itis suggested that the reactant forms a charge transfer complex at thenanotube surface, where electron donation from the latter stabilizes thetransition state and accelerates the forward rate. Once the bondsymmetry of the nanotube is disrupted by the formation of this defect,adjacent carbons increase in reactivity (FIG. 1C) and the initialselectivity is amplified as the entire nanotube is functionalized.

Carbon nanotube chemistry has been correctly described using apyramidization angle formalism [S. Niyogi et al., Acc. of Chem. Res.,2002, 35, 1105-1113]. Here, chemical reactivity and kinetic selectivityare related to the extent of s character due to the curvature-inducedstrain of the sp²-hybridized graphene sheet. Because strain energy percarbon is inversely related to nanotube diameter, this model predictssmaller diameter nanotubes to be the most reactive, with the enthalpy ofreaction decreasing as the curvature becomes infinite. While thisbehavior is most commonly the case, the role of the electronic structureof the nanotubes in determining their reactivity is increasinglyimportant—especially when desiring selectivity among a population ofsimilar-diameter CNTs (such as is often the case with SWNT product).Furthermore, because such structure is highly sensitive to chiralwrapping, chemical doping, charged adsorbates, as well as nanotubediameter, there exists a considerable diversity among these variouspathways in addition to a simple diameter dependence.

Selective Functionalization

In general, processes for selectively functionalizing carbon nanotubescomprise the steps: a) selecting a quantity of carbon nanotube material;b) suspending the carbon nanotube material in a solvent; and c) adding achemical reactant that is able to selectively functionalize the carbonnanotube material based on the electronic properties of the nanotubes.

More specifically, in some embodiments, processes for selectivelyfunctionalizing carbon nanotubes comprise the steps: a) selecting aquantity of carbon nanotube material; b) adding the carbon nanotubematerial to an aqueous surfactant solution and homogenizing to form amixture comprising surfactant-suspended carbon nanotubes; and c) addinga suitable diazonium species to the mixture in an amount which issuitable to react preferentially with the metallic and semi-metalliccarbon nanotubes, but not with the semiconducting carbon nanotubes.

Surfactants, according to the present invention, can be any chemicalagent which facilitates the dispersion of carbon nanotubes in water.Surfactants include, but are not limited to, ionic surfactants,non-ionic surfactants, cationic surfactants, anionic surfactants, sodiumdodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), sodiumoctylbenzene sulfonate, TRITON X-100, TRITON X-405,dodecyltrimethylammonium bromide (DTAB), and combinations thereof.However, organically-wrapped CNTs in an organic solvent could also bepartners for this reaction with a diazonium salt in a selectivecoupling, provided the wrapped species are single nanotubes, or smallbundles thereof, i.e., on the order of 2-3 nanotubes, such that theindividual nanotubes are accessible for the selective functionalizationprocess.

In some embodiments of the present invention, the process of forming anaqueous mixture of surfactant-suspended carbon nanotubes comprises ahomogenizing step. A homogenizing step, according to the presentinvention, can be any method which suitably homogenizes the mixture andrenders at least some of the carbon nanotubes encapsulated inmicellar-like assemblies.

In some embodiments of the present invention, the process of forming anaqueous mixture of surfactant-suspended carbon nanotubes furthercomprises ultrasonic assistance. Ultrasonic assistance can be providedby either an ultrasonic bath or an ultrasonic horn sonicator, typicallyoperating at a power from between about 200 W to about 600 W. Theduration of such ultrasonic assistance typically ranges from about 1 minto about 20 min.

In some embodiments of the present invention, the mixture ofsurfactant-suspended carbon nanotubes is centrifuged to separate thesurfactant-suspended nanotube material from other material. In suchembodiments, the other material gravitates to the bottom and thesurfactant-suspended carbon nanotubes are decanted. In some embodimentsof the present invention, the centrifugation is provided by anultracentrifuge, and centrifugation is performed with an intensity whichranges generally from about 10,000 rpm to about 90,000 rpm, and for aduration which ranges generally from about 1 hour to about 6 hour.

In some embodiments of the present invention, aryl diazonium salts areused as the diazonium species. Suitable aryl diazonium salts include,but are not limited to,

where R is selected from the group consisting of halogen, nitro, cyano,alkyl, aryl, arylalkyl, hydroxy, carboxylic ester, carboxylic acid,thiocarbonate, amide, alkoxy, polyether, polyalkyl, hydroxy alkyl, andcombinations thereof. Variations for “R” include: a) aliphatic chains orgroups for nonpolar solvent solubility; b) polystyrene, polyethylene,polypropylene, etc. for incorporation into composites or blends; c)electrically-conducting polymeric substituents (i.e., polypyrrole orpoly(phenylene vinylene)); d) polyether chain to increase water oralcohol solubility; e) carboxylic acid or carboxylate anion to increasewater solubility; f) substituents that can cross-link polymers to formcomposites; g) R can be substituted at various positions on the aromaticring (ortho, meta, para); h) there are multiple “R” groups; and, whenpresent, use of Cl, Br, and I as leaving groups to attach to a metalsurface or nanoparticle.

In some embodiments of the present invention, the aryl diazonium salt isfirst solubilized in water or another solvent, prior to adding it to themixture of surfactant-suspended carbon nanotubes, and allowing it toreact with the nanotubes. In such embodiments, a substoichiometricamount of the aryldiazonium salt is added such that it reactspreferentially with the metallic (no bandgap) and semi-metallic (“Mod 3”nanotubes (where n−m=multiple of 3) possessing a very small bandgap,sometimes referred to as a “pseudo-gap,” that is curvature induced)carbon nanotubes, but not with the semiconducting carbon nanotubes.

In some embodiments of the present invention, Raman, absorption, and/orfluorescence spectroscopies are used to used to analyze the processduring and after the reaction to indicate the reaction isselective-favoring reaction of metallic and semi-metallic nanotubesfirst.

In some embodiments of the present invention, upon completion of thepartial reaction (i.e., reaction of the metallic and semimetallicnanotubes, but not the semiconducting nanotubes), a destabilizing agentcan be added to destabilize the micellar assemblies and permitfiltration. In some embodiments, the destabilizing agent used isN,N-dimethylformamide (DMF).

Since the selective reactivity is a function of the size of the bandgap, continued addition of diazonium species will continue to reactpreferentially with the smallest band gap unreacted nanotubes present inthe mixture. As these are preferentially reacted, the reaction willshift to the nanotubes with the next larger bandgap. Ultimately, ifenough aryl diazonium salt is added, all of the nanotubes will react.

In some embodiments, however, the reaction selectivity is observed onlywith low conversion, meaning that the surface coverage of the functionalgroup is relatively small under selective conditions.

In some embodiments of the present invention, the diazonium species isgenerated in situ by reacting a substituted aniline species with analkyl nitrite (or alternatively an inorganic nitrite in the presence ofan acid). Substituted aniline species, according to the presentinvention, have the general formula

where R (the substituent, or substituents in the case of multiplesubstitutions) is selected from the group consisting of halogen, nitro,cyano, alkyl, aryl, arylalkyl, OH, carboxylic ester, carboxylic acid,thiocarbonate, amide, alkoxy, polyether, polyalkyl, hydroxyl alkyl, andcombinations thereof.

In some embodiments of the present invention, the diazonium species isgenerated in situ by reacting a dialkyltriazene with acid. Generally,any method of producing a diazonium species, or its syntheticequivalent, will work.

In some embodiments, as an alternative to dispersing the CNTs with theaid of surfactants, the CNTs are dispersed in a superacid media such asoleum. Generally, any method of dispersing CNTs, especially asindividual (unbundled) nanotubes, and that is compatible with any of thediazonium species described above, will work.

Separation of Carbon Nanotubes

In some embodiments of the present invention, the aryl diazonium saltsare selected such that they possess functional groups that are sensitiveto changes in pH of the mixture of surfactant-suspended carbon nanotubesthat have been partially reacted with said diazonium salt. In someembodiments of the present invention the diazonium salt is

where R is an OH (i.e., phenolic) group. At high pH values (e.g., >10),the OH groups are deprotonated. In embodiments where the metals andsemi-metals have been preferentially functionalized, these species canbe separated from the semiconducting carbon nanotubes usingelectrophoretic techniques like gel or capillary electrophoresis atthese high pH values.

Thus, the reaction chemistry can be carried out such that all metallicnanotubes are selectively functionalized via phenol moieties, thenseparated by electrophoretic means yielding carbon nanotubes of specifictype and which are not agglomerated in rope-like bundles. After recoveryof the fractionated material, thermal treatment of the metallicnanotubes drives off the functional groups and the resultingunfunctionalized nanotubes recover their original properties.

In some or other embodiments, changes in the solubility of CNTs ofdifferent type within a mixture of types, as a result of their selectivefunctionalization, are exploited to facilitate their separation. Forexample, to a surfactant-suspended dispersion of CNTs can be added asubstoichiometric amount of diazonium species that reacts preferentiallywith the metallic and semi-metallic CNTs to render only these typesfunctionalized. A reagent (e.g., DMF) can then be added to destabilizethe surfactant-suspension at which point the CNTs flocculate out ofsuspension. Filtration and washing of this CNT material yields a solidmixture of functionalized metallic and semi-metallic CNTs andunfunctionalized semiconducting CNTs. Dispersal of this solid product ina solvent for which the functionalizing groups have affinity allows thefunctionalized metallic and semi-metallic CNTs to be suspended, whilethe unfunctionalized semiconducting CNTs remain unsuspended. Separationcan be accomplished via centrifugation and decantation or other means.

The most immediate and obvious use of this invention is as a route tothe separation of carbon nanotubes based on their electronic structure.By selectively functionalizing metallic nanotubes, or small band gapsemiconducting nanotubes, the remaining species can, in some embodimentsof the present invention, be separated by using changes in solubilitythat come about as a result of the functionalization. The increase inmolecular weight can also be utilized for this purpose. Additionally,the functionalization can be used to selectively disrupt conduction inthe metallic and semi-metallic CNTs. Other applications includefabrication of electronic devices consisting of all metallic nanotubesfrom a starting mixture of all electronic types. The diazonium reactioncan be employed to generate highly functionalized materials.

No other method of functionalization of single-wall carbon nanotubes hasbeen shown to be selective to the electronic structure of the nanotube.This discovery is enabled by spectroscopic techniques for carbonnanotubes that have only recently become available. In particular,photoabsorption spectroscopy and fluorescence detection are employed tofollow the reaction progression and monitor the effect of substituentaddition to the nanotube electronic structure. Also, no other methodexists to uniformly functionalize carbon nanotubes in solution.Previously, functionalized nanotubes consisted of highly functionalizednanotubes and unfunctionalized nanotubes. This observation wasattributed to the bundling that occurs with nanotubes in the solidstate.

EXPERIMENTAL EXAMPLES

The following examples are provided to more fully illustrate some of theembodiments of the present invention. It should be appreciated by thoseof skill in the art that the techniques disclosed in the examples whichfollow represent techniques discovered by the inventors to function wellin the practice of the invention, and thus can be considered toconstitute exemplary modes for its practice. However, those of skill inthe art should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments that are disclosed andstill obtain a like or similar result without departing from the spiritand scope of the invention.

Example 1

This Example serves to illustrate the selective reaction ofsurfactant-suspended CNTs with diazonium species in accordance with someembodiments of the present invention.

Micelle-coated (surfactant-suspended) single-wall carbon nanotubes aregenerated via homogenation of raw material and 1% of sodium dodecylsulfate in water or deuterium oxide (D₂O) for 1 hour, followed bysonication for 10 minutes. The solution is then centrifuged for 4 hoursand decanted to generate the micelle-coated nanotubes. The pH is thenadjusted with 1.0 N NaOH to approximately 10, and one of a variety ofdiazonium salts is added to the aqueous solution/suspension. Thediazonium salt can be added as a solid directly to the decantedmaterial, or the diazonium salt can be dissolved in water or D₂O andthen added as a dilute solution. When a large excess of the salt isadded, selectivity is not observed, but all the nanotubes arefunctionalized to a high degree. For selective functionalization, adilute solution of the salt is prepared by solubilizing the diazoniumsalt in water or D₂O (roughly 1.5 M), and an aliquot (roughly 5 μL) ofthis solution is added to the nanotube decants with stirring. Thereaction can be monitored by several spectroscopic techniques in orderto determine the extent of functionalization. Once the functionalizationis complete, the reaction mixture is diluted with some organic solvent(e.g., acetone, DMF), and the flocculated nanotubes are then collectedby filtration over a polytetrafluoroethylene (PTFE) membrane. Thecollected solid is then washed with acetone and water to removeunreacted diazonium salt, diazonium decomposition side-products, andsodium dodecyl sulfate. The nanotube sample is then collected from themembrane and dried in a vacuum oven at 60° C.

The description here is not meant to be limiting. There are variationsin concentration and reaction times, as well as methods for generatingthe intermediates that could be made. For example, one could generatethe diazonium salts in situ from an aniline and an alkyl nitrite or ananiline and sodium nitrite/acid. Furthermore, the diazonium salts thatrespond best, to date, are aryldiazonium salts, however, this should notbe construed as a limitation. Functional groups or substituents on thearyl ring can be varied to modify the hydrophilic and hydrophobiccharacter of the nanotube addends to enhance separation efficacy orother properties.

Example 2

This Example serves to illustrate how selective functionalization can befollowed with absorption spectroscopy.

The evidence for selective functionalization can be observed in theultraviolet-visible-near infrared (UV-vis-NIR) absorption spectrum ofthe solution during and after the reaction. The reaction at the nanotubesurface necessarily disrupts the photoexcitation process that normallygives the nanotube a prominent and sharp absorption maximum in thisspectrum. FIG. 2 shows that nanotubes having such a maximum at longerwavelengths (lower energy band gaps) are affected disproportionately atlower concentrations as their peaks decay. Referring to FIG. 2, (A)UV-vis-NIR spectra of sodium dodecyl sulfate-suspended carbon nanotubesafter the addition of various amounts of 4-chlorobenzenediazoniumtetrafluoroborate (in mol/1000 mol carbon), and wherein (B) is anexpanded view of the metallic region, wherein the peaks a-f,corresponding to 0.0, 2.1, 3.9, 5.6, 9.1, and 11.8 side groups per 10³nanotube carbons, respectively, are seen to decrease with increasingside group concentration. Thus, it is seen that smaller diameternanotubes remain unaffected until larger reagent concentrations.

Under carefully controlled conditions, the above-described chemicalbehavior of CNTs can be exploited to obtain highly selectivefunctionalization of metallic and semi-metallic nanotubes to theexclusion of the semiconductors. In one such embodiment, a recirculatingflow reactor was used to transfer 150 mL/min of sodium dodecyl sulfatesuspended carbon nanotubes through a cuvettes with inlet and outletports. To monitor this reaction in situ, continuous UV-vis-NIR spectrawere generated after the addition of a metered amount of diazonium arylchloride tetrafluoroborate. Additions were made in 0.05 mM incrementsafter the system reached a steady state condition. FIGS. 2A and 2B showthe UV-vis-NIR absorption spectra of aqueous suspended nanotubes aftersuccessive additions of 4-chlorobenzenediazonium tetrafluoroborate aftersteady state. The spectrum monitors the v1→c1 electronic transitions ofthe metallic and semi-metallic nanotubes from roughly 440 to 645 nm aswell as the v1→c1 and v2→c2 of the semiconducting nanotubes in theranges from 830 to 1600 nm and 600 to 800 nm respectively. Theseseparated absorption features allow for the monitoring of valenceelectrons in each distinct nanotube; as the species reacts to formcovalent linkages, electrons are localized and these maxima decay. InFIG. 2, it can be seen that under such controlled additions, onlymetallic transitions initially decay, indicating a highly preferentialfunctionalization of metallic nanotubes (note that in FIG. 2B, the peaksdecrease with increasing side group concentration). This selectivity isremarkable given that these transitions arise from electrons that aremuch lower in energy compared to the v1→c1 and v2→c2 transitions of thesemiconductors. Indeed, the selective decay of these metallictransitions is unprecedented, and identifies this process as distinctfrom reversible electronic withdraw [Strano et al., Journal of PhysicalChemistry B, 2003, 107, 6979-6985] or generic “doping” processes [Itkiset al., Nanoletters, 2002, 2, 155-159] as has been previously reported.

Example 3

This Example serves to illustrate how selective functionalization can befollowed spectroscopically with Raman spectroscopy.

FIG. 3 shows the Raman spectrum at 532 nm excitation of the samesolution after 0.05 mM reagent added. FIG. 3A shows the low Raman shiftregion that normally possesses peaks representative of distinct nanotubediameters that are resonant with the laser. Only one is visible (thelowest wavelength transition of the group as indicated.) FIG. 3B showsthat the “D-band” has increased—a characteristic of functionalizationbut the largest band-gap nanotubes (also shown) still fluoresceindicating the absence of functionalization (unperturbed electronictransitions). All of this takes place at constant bulk pH=10.

More specifically, this reaction selectivity is confirmed by thepreservation of band-gap fluorescence of the semi-conducting nanotubes,which is known to be highly sensitive to chemical defects. Referring toFIG. 3, (A) Raman spectrum at 532-nm excitation, showing the growth ofthe “disorder” mode with increasing functionalization from 0 (i) to 5.6(ii) to 22.4 (iii) groups attached per 1000 carbon atoms; wherein (B)the intensity of the tangential mode (TM)×0.1 decreases as resonanceenhancement of the scattering event is lost with increasing reaction;and wherein the disorder mode, D, increases sharply then decays becauseof the same loss of enhancement. The functionalization increases theintensity of a phonon mode at 1330 cm⁻¹ (D-band) in the Raman spectrumas shown in FIG. 3A at 532 nm excitation. Its presence confirms theconversion of an sp² C to an sp³ C on the nanotube during the formationof an sp³ C-sp² C nanotube-aryl bond. This mode increases sharply withincreasing functionalization, then decreases along with the C—Ctangential mode (“TM-peak”) as the system loses its electronic resonance(FIG. 3B). These results allow, for the first time, a spectroscopiccorrelation of the number of sidewall functionalization events to thisphonon intensity at low conversion, and will be valuable for the controlof nanotube sidewall chemistry. The addition of the moiety to thesidewall of the nanotube disrupts the radial phonon that gives rise tolow frequency Raman lines distinct for species of a particular diameterwhich causes the mode to decay accordingly as the particular (n,m)nanotube reacts. FIG. 4 analogously shows the solution phase Ramanspectra at 532 nm of the mixture with each reactant addition aftersteady state and the relative rates of the decays of these featuresreveals unprecedented reactivity differences between chiralsemi-metallic species. Here, Raman spectroscopy probes nanotubes withnearly identical transition energies and these differences reveal acurvature dependent stabilization of the charge transfer complex thatmay ultimately be exploited to separate semi-metallic and metallicspecies. Referring to FIG. 4, (A) low wavenumber Raman spectra at 532-nmexcitation of the starting solution, wherein four metallic nanotubes[(13,1), (9,6), (10,4) and (9,3)] are probed at this wavelength and onesemiconductor (9,2) via a radial mode sensitive to nanotube diameter,wherein (B) after 5.6 groups attached per 1000 carbons,functionalization disrupts this mode, as seen by the decay particularlyof the small-diameter metals, and providing initial evidence ofselective reactivity among metals provides a handle for separation ofthese species, and wherein (C) after a ratio of 22.4, all metallic modeshave decayed, leaving only the single semiconductor, in agreement withFIG. 2B. It is noted that when all v1→c1 transitions of semi-metallicand metallic species have decayed (FIG. 2), only one low-frequency Ramanmode that has been previously assigned to the (9,2) semiconductor[Strano et al., Journal of Physical Chemistry B, 2003, 107, 6979-6985]remains unaffected. This serves as the first independent confirmation ofthe recent spectroscopic assignment of these features [Bachilo et al.,Science, 2002, 298, 2361; M. S. Strano, Nanoletters, 2003, 3, 1091].

Example 4

This Example serves to illustrate how CNTs can be separated by type viaselective functionalization.

Selective functionalization as a handle for nanotube separations isunique in that it allows manipulation independent of tube length, unlikemost chromatographic-based methods. Because the selectivity is nearlycomplete, this chemistry can form the basis for high efficiencyseparations in contrast to the minor enrichments that have been reportedto date [Chattopadhyay et al., J. Am. Chem. Soc., 2003, 125, 3370-3375;Zheng et al., Nature Materials, 2003, 2, 338-342]. Applicants havephenolated the sidewalls of metallic nanotubes with approximately 0.11sidegroups per carbon and fractionated samples using electrophoreticmeans. Above a pH of 10.2, these phenol groups are deprotonated leavinga net negative charge per group on the nanotube (FIG. 6A). A non-ionicsurfactant was used in this case to enhance the electrostatic changesupon functionalization. The change in electrophoretic mobility, μ, wasmeasured upon reaction using migration velocities during capillaryelectrophoresis (CE). This mobility is the observed velocity, v,normalized to the field strength across the capillary, E, and equal to:μ=(v/E)=(q/f)where q is the net charge on the nanotube and f is a hydrodynamicresistance factor strongly dependent upon the length to diameter ratio(L/D) of the nanotube. The functionalization does not alter f since thelength is unaffected, and the diameter of the tube is extended far lessthan the surfactant-adsorbed layer on the sidewall. However, themobility is sensitive to charged groups at the nanotube surface.Unfunctionalized nanotubes in TRITON X-405 consistently show 3 distinctpopulations when fractionated by an applied electric field: those withδ⁺ charge from the adsorption of the cationic buffer molecules, thosethat are neutral, and those with δ⁻ charge from surface —OH and —COOHgroups on the sides and ends of the tubes from processing. Partitioningbetween these three groups depends on the balance of cationic adsorptionand anionic functionalities. FIG. 6B is a CE trace of unfunctionalizedand phenol-functionalized material showing differences in migrationtimes of 2 min. Deuterium oxide provides a neutral marker with speciesmigrating later than this time being negatively charged. Scaling of themigration velocity by the applied field allows for a comparison ofelectrophoretic mobility distributions (towards the positive electrode)between reacted and unreacted nanotubes. In FIG. 6C, this comparisondemonstrates how functionalized material is extracted from the totalpopulation by exploiting this change in mobility due to the negativecharge.

Example 5

This Example serves to illustrate how selectively functionalized CNTscan be made to revert back to their unfunctionalized, pristine state.

Thermal pyrolysis of the reacted material at 300° C. in an atmosphere offlowing inert gas cleaves the aryl moieties from the sidewall andrestores the spectroscopic signatures of the aromatic, pristinenanotubes [Bahr et al., J. Mat. Chem., 2002, 12, 1952-1958]. FIG. 5compares the Raman spectra before (solid line) and after (dotted line)recovery and thermal pyrolysis at 633 nm (FIG. 5). This wavelength wasemployed because it probes a mixture of metals and semiconductors forsamples prepared by CO disproportionation [Strano, Nanoletters, 2003, 3,1091]. Thus, the radial phonon modes are nearly completely restoredafter thermal treatment. Similarly, electronic transitions in theabsorption spectrum are restored indicating the loss of the side groupand a restoration of the original electronic structure of the nanotube.The reversibility of the chemistry implies that intrinsic electronic andoptical properties of the pristine nanotubes can be recovered. Hencethis selective chemistry can be used as a reversible route to separate,deposit or chemically link nanotubes of a particular electronicstructure and the original optical and electronic characteristics canthen be recovered.

In summary, diazonium reagents are shown to functionalize single walledcarbon nanotubes suspended in aqueous solution with high selectivity andenable manipulation according to electronic structure. For example,metallic species can be reacted to the near exclusion of semiconductingnanotubes under controlled (e.g., substoichiometric) conditions.Selectivity is dictated by the availability of electrons near the Fermilevel to stabilize a charge transfer transition state preceding bondformation. The utility of this chemistry as a means of manipulatingsingle-wall carbon nanotubes by their electronic structure isdemonstrated by the selective attachment of a phenol moiety andsubsequent separation using electrophoretic means. The chemistry can bereversed using a thermal treatment that restores the pristine electronicstructure of the nanotube.

All patents and publications referenced herein are hereby incorporatedby reference. It will be understood that certain of the above-describedstructures, functions, and operations of the above-described embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention as defined by the appendedclaims.

1. A method for separating carbon nanotubes based on their electronictype, said method comprising: a) suspending a plurality of carbonnanotubes in a solvent to form a plurality of suspended carbonnanotubes; wherein the plurality of suspended carbon nanotubes comprisemetallic carbon nanotubes, semimetallic carbon nanotubes andsemiconducting carbon nanotubes; b) reacting a substoichiometric amountof a functionalizing species with the plurality of suspended carbonnanotubes such that the metallic carbon nanotubes and the semimetalliccarbon nanotubes, but not the semiconducting carbon nanotubes, reactwith the functionalizing species to form a mixture comprisingfunctionalized metallic carbon nanotubes, functionalized semimetalliccarbon nanotubes and unfunctionalized semiconducting carbon nanotubes;wherein the substoichiometric amount of the functionalizing species iswith respect to the amount of carbon comprising the carbon nanotubes;and wherein the substoichiometric amount of the functionalizing speciesis selected such that a reaction of the functionalizing species occurswith the metallic carbon nanotubes and the semimetallic carbon nanotubesbut not with the semiconducting carbon nanotubes; and c) separating thefunctionalized metallic carbon nanotubes and the functionalizedsemimetallic carbon nanotubes from the unfunctionalized semiconductingcarbon nanotubes.
 2. The method of claim 1, wherein the functionalizingspecies is a diazonium species.
 3. The method of claim 2, wherein thediazonium species is an aryl diazonium salt.
 4. The method of claim 3,wherein the aryl diazonium salt is generated in situ by reacting asubstituted aniline species with an alkyl nitrite.
 5. The method ofclaim 3, wherein the aryl diazonium salt has a structure

wherein R is selected from the group consisting of halogen, nitro,cyano, alkyl, aryl, arylalkyl, OH, carboxylic ester, carboxylic acid,thiocarbonate, amide, alkoxy, polyether, polyalkyl, hydroxyl alkyl, andcombinations thereof; and wherein R can be substituted at variouspositions on the aromatic ring and there can be multiple R groups on thearomatic ring.
 6. The method of claim 5, wherein R is OH.
 7. The methodof claim 1, wherein the functionalized metallic carbon nanotubes and thefunctionalized semimetallic carbon nanotubes comprise phenolic OHmoieties.
 8. The method of claim 7, wherein the functionalized metalliccarbon nanotubes, the functionalized semimetallic carbon nanotubes andthe unfunctionalized semiconducting carbon nanotubes are suspended in asurfactant solution.
 9. The method of claim 8, further comprising:deprotonating at least a portion of the phenolic OH moieties by raisingthe pH of the surfactant solution.
 10. The method of claim 9, whereindeprotonating comprises raising the pH of the surfactant solution above10.
 11. The method of claim 9, wherein separating comprises anelectrophoresis technique.
 12. The method of claim 11, wherein theelectrophoresis technique is selected from the group consisting of gelelectrophoresis, capillary electrophoresis, and combinations thereof.13. The method of claim 1, further comprising: after separating,thermally defunctionalizing the functionalized metallic carbon nanotubesand the functionalized semimetallic carbon nanotubes to formdefunctionalized metallic carbon nanotubes and defunctionalizedsemimetallic carbon nanotubes.
 14. A method for separating carbonnanotubes based on their electronic type, said method comprising: a)providing a plurality of functionalized carbon nanotubes suspended in asolution; wherein the plurality of functionalized carbon nanotubescomprise functionalized metallic carbon nanotubes, functionalizedsemimetallic carbon nanotubes, and unfunctionalized semiconductingcarbon nanotubes; wherein the functionalized metallic carbon nanotubesand the functionalized semimetallic carbon nanotubes comprise phenolicOH moieties; b) raising the pH of the solution above 10 such that atleast a portion of the phenolic OH moieties become deprotonated; and c)after raising the pH, separating the functionalized metallic carbonnanotubes and the functionalized semimetallic carbon nanotubes from theunfunctionalized semiconducting carbon nanotubes.
 15. The method ofclaim 14, wherein separating comprises an electrophoresis techniqueselected from the group consisting of gel electrophoresis, capillaryelectrophoresis, and combinations thereof.
 16. The method of claim 14,further comprising: after separating, thermally defunctionalizing thefunctionalized metallic carbon nanotubes and the functionalizedsemimetallic carbon nanotubes to form defunctionalized metallic carbonnanotubes and defunctionalized semimetallic carbon nanotubes.
 17. Themethod of claim 16, wherein thermally defunctionalizing comprisesheating the functionalized metallic carbon nanotubes and thefunctionalized semimetallic carbon nanotubes at 300° C.
 18. The methodof claim 13, wherein thermally defunctionalizing comprises heating thefunctionalized metallic carbon nanotubes and the functionalizedsemimetallic carbon nanotubes at 300° C.
 19. The method of claim 1,wherein separating comprises suspending the functionalized metalliccarbon nanotubes and the functionalized semimetallic carbon nanotubes ina solvent in which the unfunctionalized semiconducting carbon nanotubesremain unsuspended.
 20. The method of claim 1, wherein the solventcomprises a superacid.
 21. A method for separating carbon nanotubesbased on their electronic type, said method comprising: a) suspending aplurality of carbon nanotubes in a surfactant solution to form aplurality of suspended carbon nanotubes; wherein the plurality ofsuspended carbon nanotubes comprise metallic carbon nanotubes,semimetallic carbon nanotubes and semiconducting carbon nanotubes; b)reacting a substoichiometric amount of a diazonium species with theplurality of suspended carbon nanotubes such that the metallic carbonnanotubes and the semimetallic carbon nanotubes, but not thesemiconducting carbon nanotubes, react with the diazonium species toform a mixture comprising functionalized metallic carbon nanotubes,functionalized semimetallic carbon nanotubes and unfunctionalizedsemiconducting carbon nanotubes; wherein the substoichiometric amount ofthe diazonium species is with respect to the amount of carbon comprisingthe carbon nanotubes; and wherein the substoichiometric amount of thediazonium species is selected such that a reaction of the diazoniumspecies occurs with the metallic carbon nanotubes and the semimetalliccarbon nanotubes but not with the semiconducting carbon nanotubes; andc) separating the functionalized metallic carbon nanotubes and thefunctionalized semimetallic carbon nanotubes from the unfunctionalizedsemiconducting carbon nanotubes.
 22. The method of claim 21, wherein thediazonium species comprises an aryl diazonium salt.
 23. The method ofclaim 22, wherein the aryl diazonium salt has a structure

wherein R is selected from the group consisting of halogen, nitro,cyano, alkyl, aryl, arylalkyl, OH, carboxylic ester, carboxylic acid,thiocarbonate, amide, alkoxy, polyether, polyalkyl, hydroxyl alkyl, andcombinations thereof; and wherein R can be substituted at variouspositions on the aromatic ring and there can be multiple R groups on thearomatic ring.
 24. The method of claim 21, wherein the functionalizedmetallic carbon nanotubes and the functionalized semimetallic carbonnanotubes comprise phenolic OH moieties.
 25. The method of claim 24,further comprising: deprotonating at least a portion of the phenolic OHmoieties by raising the pH of the surfactant solution.
 26. The method ofclaim 25, wherein deprotonating comprises raising the pH of thesurfactant solution above
 10. 27. The method of claim 25, whereinseparating comprises an electrophoresis technique selected from thegroup consisting of gel electrophoresis, capillary electrophoresis, andcombinations thereof.
 28. The method of claim 21, further comprising:after separating, thermally defunctionalizing the functionalizedmetallic carbon nanotubes and the functionalized semimetallic carbonnanotubes to form defunctionalized metallic carbon nanotubes anddefunctionalized semimetallic carbon nanotubes.
 29. The method of claim28, wherein thermally defunctionalizing comprises heating thefunctionalized metallic carbon nanotubes and the functionalizedsemimetallic carbon nanotubes at 300° C.
 30. The method of claim 21,wherein separating comprises suspending the functionalized metalliccarbon nanotubes and the functionalized semimetallic carbon nanotubes ina solvent in which the unfunctionalized semiconducting carbon nanotubesremain unsuspended.